The invention relates to a strain-sensitive resistor comprising a resistance layer arranged on a support element and an electromechanical transducer produced with this resistor.
The as yet unpublished German patent application 197 47 001.7 discloses a strain-sensitive resistor in which the resistor is arranged on a shaft constructed as a support element. The shaft is subjected to mechanical loading, the surface strain resulting from this being picked off by the strain-sensitive resistor arranged on this shaft without any intermediate support. The resistance layer is applied to the shaft in the form of a resistive paste, using a printing technique, and intimately connected to said shaft after a heat treatment.
In the event of torsion of the shaft, two main strains at 45xc2x0 are formed on the surface of the shaft. These strains are evaluated in order to determine the torque. In this case, the main strains have the same magnitude but the opposite sign.
The thick-film resistance pastes used have positive strain factors (K factors) both for the longitudinal and for the transverse strains, for which reason only the difference of these two factors can be used to determine the effective resistance change. The signal voltages registered at the torque sensor are therefore very low and must be amplified by a comprehensive electrical arrangement, as a result of which the influence of interference on the small measured signal is increased.
The invention is therefore based on the object of specifying a strain-sensitive resistor in which an increase in the measured signal picked off across the resistor can be implemented in a simple way.
According to the invention, the object is achieved by the support element having a recess on its surface which, when the support element is subjected to mechanical stress in at least one area of the surface of the support element in which the resistance layer is positioned, produces a ratio between the two main strains in the resistance layer which differs in magnitude.
The advantage of the invention resides in the fact that the signal response of the sensor is increased simply, without any complex change to the shaft geometry. Such a sensor is suitable for mass production, since it can be produced cost-effectively and quickly.
Because of the recess, the mechanical stresses acting on the support element, such as torsion and flexure, are superimposed, the strains in the main directions (longitudinal, transverse) having unequal magnitudes.
This recess can be produced by changes which can be implemented simply on the support element, such as drilled holes, notches and slits, so that the two main strain directions on the surface of the support element on account of torsions no longer have the same magnitudes.
The recess is advantageously formed as a continuous opening in the support element.
In a development, the opening is formed as a slot, the resistance layer being arranged in the vicinity of the radial area of the slot.
The strain-sensitive resistor can be produced particularly simply if the resistance layer is arranged on a planar surface of the support element.
If the support element of the resistance layer at the same time constitutes the component to be stressed mechanically by torsion, it is possible to dispense with an intermediate support between the strain-sensitive resistor and the component to be loaded. The mechanical loading to be detected is in this case picked off directly from the component to be loaded, without signal distortions produced by the intermediate support being produced. Such a resistor reduces the production costs considerably.
A reliable, nondetachable connection is achieved if the component to be loaded mechanically and the resistance layer are connected to each other via an intimate connection, for example are sintered. This is achieved by the paste-like resistance layer applied to the support element by a printing technique being sintered to the mechanical component during a high-temperature process.
In a development, the support element is electrically conductive, an insulating layer being arranged between the resistance layer and the support element. Such a configuration is particularly practical if the component to be loaded mechanically consists of metal, such as is the case, for example, when it is used a torque sensor in power steering systems. In this way, short circuits on the sensor can reliably be prevented.
In a refinement, the insulating layer is paste-like and is applied to the support element before the application of the resistance layer, in a self-contained high-temperature process, or together with the resistance layer, during a high-temperature process.
As an alternative to this, the insulating layer, as described, is sintered to the support element, either independently or together with the resistance layer, if the insulating layer is film-like.
In this case, the insulating layer enters into an intimate connection with the component to be loaded. This connection can be implemented by a reliable process and is extremely stable in the long term.
In particular, the production of the strain-sensitive resistance with a film-like insulation layer permits the application of the strain gage to a component with a nonplanar surface.
In a method of producing the strain-sensitive resistor, in which the insulating layer and the resistance layer are applied to the support element one after another, the film-like insulating layer bearing the resistance layer in at least one area is approximately matched to the shape of the recess in the surface of the support element, this area being applied to the surface of the support element so as to be congruent with this shape of the recess and subsequently being subjected to the action of heat.
This has the advantage that the insulating layer is used as an adjustment aid at the same time during the subsequent application of the resistor structure to the support element, and it is therefore ensured that the resistance layer is arranged in that area of the support element where the greatest difference occurs between the longitudinal and transverse strain.
In a further development of the invention, the insulating layer and/or the resistance layer are arranged on a support sheet, after which the side of the support sheet that bears the insulating and/or resistance layer is covered with a flexible film layer, whose adhesion to the insulating layer and/or resistance layer is greater than the adhesion of the support sheet to the insulating layer and/or resistance layer, at least the film layer being matched in one area to the shape of the recess in the surface of the support element, and the film layer with the insulating layer and/or resistance layer being applied to the support element in such a way that the correspondingly shaped areas of the recess and the film layer are aligned congruently, the support element subsequently being subjected to the action of heat to burn out the film layer and sinter on the insulating layer and/or resistance layer.
The advantage of the invention resides in the act that the existing structure is produced on a support material in the form of the support sheet and, after production, is placed on the support element with the aid of the transport film in the manner of a transfer. In this case, too, the shape of the transport film makes the adjustment of this arrangement on the support element easier. On the basis of this procedure, the desired layer structure can be transferred onto any conceivable geometric shape of the support element and can be sintered to form a firmly adhering layer during the subsequent heat treatment.
In the case of this design, both the insulating layer and the resistance layer are arranged on a single support sheet, and are transported with only one film layer.
In this way, a resistor is produced which adheres reliably to nonplanar surfaces of support elements, even under long-lasting mechanical and thermal loading. This is advantageous in particular when the support element is a component to be loaded mechanically, to which layers are applied by sintering.
The insulating layer and/or the resistance layer are advantageously applied to the support sheet by a printing technique and dried. It is therefore possible not only to apply simple unstructured layers but also structured structures such as entire resistance networks to a nonplanar surface of a support element.
Using such a production method, it is possible to produce rolling structures which have dimensions determined by a computer and which are given their necessary geometric structure and dimensions only when applied to the nonplanar surface.
In one configuration, the insulating layer is printed onto the support sheet in the form of a glass frit. After the glass frit has been dried, a conductive paste is applied to the insulating layer as the resistance layer and dried, the film layer then being applied in the form of a synthetic resin film.
As an alternative to this, an insulating layer, arranged on a first carrier sheet and dried, is applied to the support element by means of the film layer and subjected to the action of heat, and then the resistance layer, printed onto a second support sheet and dried, is positioned on the already heat-treated insulating layer with the aid of a second film layer arranged on said resistance layer and is then likewise heat-treated.
The method has the advantage that, depending on the application, both the entire structure can be produced on a support sheet and, by means of a single film layer, can be transported from the support sheet to the support element, or else each layer of the structure is produced individually on a support sheet. The individually produced layer is likewise positioned on the support element by means of a film.
In another development of the invention, an electromechanical transducer has a device with strain-sensitive resistors which comprises a resistance layer arranged on a common support element, the resistance layer and the support element being separated by an insulating layer and it being possible for an electrical signal corresponding to the strain to be picked up across the resistors. In this case, evaluation electronics for the electrical signal corresponding to the strain are arranged on the support element, the support element additionally having a recess on its surface which, when the support element is subjected to mechanical stress in at least one area of the surface of the support element, in which at least one strain-sensitive resistor is positioned, produces a ratio between longitudinal and transverse strain which differs in magnitude.
The invention has the advantage that both the sensor element and the sensor electronics are applied directly to the component to be loaded mechanically.
In a refinement, the strain gages and the structure of the evaluation electronics, such as conductor tracks, contact points, thick-film resistors, are arranged on a common, film-like insulating layer, which is then centered jointly onto the component to be loaded mechanically.
This production of sensor element and sensor electronics even permits arrangement on components which do not have a planar surface, for example on round components.